1. Field of the Invention
This invention relates to an electrophotographic photosensitive member to be applied for, particularly, the so called laser copying machine, laser color copying machine and laser printer for obtaining a copied image of high gradation degree which is 100 .mu.m or less in spot diameter of laser and obtained by performing reproduction of half tone in the technical field in which image formation is effected with laser beam.
Also, the present invention relates to an electrophotographic photosensitive member which comprises performing reproduction of half tone by varying the laser dose in two or more stages.
Further, the present invention relates to an electrophotographic photosensitive member to be applied for a higher gradation, higher image quality laser copying machine, laser color copying machine and laser printer, which comprises performing reproduction of half tone by varying the pulse wave width of laser beam (PMW method).
2. Related Background Art
In the electrophotographic devices of the prior art, the so called analog system was predominant, which performed latent image formation by irradiating a light source in general such as halogen light, fluorescent light on a manuscript and irradiating the reflected light on an electrophotographic photosensitive member (image exposure).
On the other hand, it is well known in the art that with the progress of development of the so called digital system electrophotographic device by use of laser beam, LED beam, liquid crystal shutter, etc. as the light source, the laser beam printer has not assumed dominance as demanded from the printers for computer, facsimile, etc.
Above all, quite recently, by use of a digital light source such as laser, etc., as further advanced from the line printer of the prior art, research and developments have been actively performed regarding a laser copying machine for performing image copying.
The greatest difference between laser copying machine and laser beam printer resides in gradation reproducibility. Since copying of photography or image is performed in a laser copying machine, high half tone reproduction, high image quality and high resolution are demanded. For reproduction of half tone, it is performed by increase or decrease of number of dots, but according to this system, there is a limit in gradation reproduction, and also coarseness, etc. are conspicuous on image quality. Photographic copying of high image quality and high resolution cannot be obtained under the present state of the art.
For obtaining half tone of high quality by solving such problems, there are three means as shown below.
The first means is to narrow the laser spot diameter, thereby increasing previously the dot number. In the prior art, 240 dpi was primarily employed, but 300, 400 dpi are becoming more popular in recent years. Accordingly, the laser spot diameter, which was previously 120 .mu.m or more, is now becoming 100 .mu.m or less, particularly 70 .mu.m or less.
The second means is to reproduce half tone by varying the laser quantity in two or more stages. Practically, it is difficult to obtain a high degree of half tone only by quantity change, and it is frequently practiced to use this means in combination with the first means for changing the dot number.
The spot diameter as herein mentioned is represented by the width with the height of 1/e.sup.2 for the peak value of the laser emission distribution showing a Gaussian distribution. When the cross-section of the spot is not completely spherical, it is defined as the maximum diameter.
The third means is the PWM (Pulse Width Modulation) system. As contrasted to the system of the prior art which performs reproduction of half tone by increase and decrease of dot number (FIG. 1A), a new system for obtaining a copy of high gradation, high image quality is by laser modulation of the PWM system as already disclosed by the present applicant in Japanese patent application No. 190659/1986. That system is going to be newly developed.
In essence, the PWM system is a technique to reproduce half tone by varying the size (laser spot diameter) of a dot without varying the dot number as shown in FIG. 1B. According to this system, high gradation similar to analog image can be obtained for the first time, and also it has become possible to obtain a copy of high quality without coarseness.
A laser copying machine of the PWM system can exhibit its effect in copying of photographs, and it is a particularly effective technique in a laser color copying machine. Also, it has not only reached a level comparable to the analog system in image quality, but also has a number of excellent copying characteristics, correction of image quality and tone, control, conversion, transfer, various editing functions, etc.
As a matter of course, this system is applicable also for a laser beam printer which can also effect reproduction of half tone in place of a laser beam printer as the line printer of the prior art.
As a photosensitive member for electrophotography, inorganic photoconductive members such as of silicon, selenium, cadmium sulfide, zinc oxide, etc. have been heretofore known.
Further, since discovery of a specific compounds exhibiting photoconductivity, a large number of organic photoconductors have been developed. For example, there have been known organic photoconductive polymers such as poly-N-vinylcarbazole, polyvinylanthracene, etc., low molecular weight organic photoconductive materials such as carbazole, anthracene, pyrazolines, oxadiazoles, hydrazones, polyarylalkanes, etc., organic pigments and dyes such as phthalocyanine pigments, azo pigments, cyanine dyes, polycyclic quinone pigments, perylene type pigments, indigo dyes, thioindigo dyes or squaric acid methine dyes, etc. Particularly, since organic pigments and dyes have photoconductivity can be synthesized more easily as compared with inorganic materials, and yet expanded in variation to select compounds exhibiting photoconductivity in appropriate wavelength regions, a large number of photoconductive pigments and dyes have been proposed. For example, there have been known electrophotographic photosensitive members by use of disazo pigment exhibiting photoconductivity as the charge generation substance in the photosensitive layer separated into the functions of charge generation layer and charge transport layer as disclosed in U.S. Pat. Nos. 4,123,270, 4,247,614, 4,251,614, 4,256,821, 4,260,672, 4,268,596, 4,278,747, 4,279,981, 4,293,628, 4,356,243, 4,436,800, 4,471,040, 4,582,771, etc.
An electrophotographic photosensitive member by use of such organic photoconductors can be produced by coating, and therefore is very high in productivity and can provide inexpensive photosensitive members, and also has the advantage of controlling freely the photosensitive wavelength region by selection of the organic pigment.
Among them, a laminated type photosensitive member obtained by lamination of a charge transport layer and a charge generation layer composed mainly of a charge generation material is more excellent in residual potential, memory, repeated characteristic, etc. than other simple layer type photosensitive member, and has the advantage particularly in improvement of sensitivity.
In recent years, organic photoconductive members have not only reached a level comparable with high sensitivity inorganic photosensitive member such as a-Se, a-Si, etc. at least in aspect of sensitivity, but also some of them have already surpassed inorganic photoconductive members in sensitivity particularly in the wavelength region (770-800 nm) of the solid laser light source generally employed today.
For the reasons as mentioned above, for electrophotographic devices by use of laser beam, organic photoconductors tend to be increasingly used year by year.
However, when an organic photoconductor is used in an electrophotographic apparatus by use of a laser capable of reproducing half tone, particularly an electrophotographic apparatus by employment of the PWM system (copying machine particularly color copying machine, printer capable of reproducing half tone), there is a substantial problem not found in the prior art, which has become an obstacle in practical application.
When an organic photosensitive member is set on a laser color copying system of the reversal developing system, the photosensitive member is stopped after completion of copying and left to stand for a while, the portion corresponding to that immediately below the corona charger is damaged, and there is generated a phenomenon that the image corresponding to that site is dropped out in white on copying.
This phenomenon was found to be very marked in the case of an electrophotographic apparatus using a laser as the light source in which importance is attached to reproduction of half tone, further a laser color copying machine in which reproduction of the image at the low contrast site (highlight portion) is demanded, particularly a copying machine or color copying machine of the PWM system, and above all a laser color copying machine in which copying is effected by repeating developing for 4 times and synchronizing the position of the photosensitive drum with the image exposure position. Further, it was also found that white drop-out became more marked as the progress of successive copying progressed to the extent that the desired image was unavailable.